June 15, 1905J 



NA TURE 



161 



At the same time there arises here a case of the optical 

 influence of isomerism, for hexylene, which has already 

 been mentioned, with the same formula (CjH,„) as hexa- 

 meth\lene, but in structure an olefinc : — 



CH2 = CKCH., CH2CH2 CH., 



possesses the familiar refractive increment of 2 units. 

 This example again shows how the spectrochemical be- 

 haviour of a body discloses its chemical structure by 

 enabling us to distinguish with certainty between an 

 optically normal cycloid (or ring-substance) and an 

 isomeric open-chain olefinoid formation, which is optically 

 abnormal. 



Carbon can thus act variously upon light according to 

 the manner in which its atoms are combined. We can 

 therefore transfer the refractive increment of the double 

 bond to the atom itself. 



In the diamond, and in all paraffinoid carbon compounds, 

 the atomic refraction of carbon equals 5 : it is therefore equal 

 to 10 for two carbon atoms. The double bond increases 

 the refraction by 2, so that for two carbon atoms with 

 a double bond the refraction amounts to 12. The atomic 

 refraction of one carbon atom with a double bond is there- 

 fore equal to 6, i.e. 20 per cent, greater than that of the 

 ;itom with the single bond : — 



Atomic 

 Refraclion 



1 Carbon atom C (diamond and paraffins) 5 



2 Carbon atoms 2C (diamond and paraffins) . 10 

 Double bond 2 



2 Carbon atoms with a double bond (C=C ... 12 



I Carbon atom with a double bond (C=': - 6 



Carbon, being a quadrivalent element, can also appear 

 with iridic bonds : — 



R.C = C.R 



Experiment has shown that carbon with a triple bond also 

 acquires a special atomic refraction. 



Thus it becomes possible to establish the presence of 

 this kind of bond in substances, and to distinguish it from 

 the double and simple bonds — a further criterion of struc- 

 ture. 



In consequence of these discoveries it became highly 

 probable that all multivalent elements, such as carbon, 

 possessed an atomic refraction varying with the kind of 

 bond, while the univalent elements, such as hydrogen, 

 display constant optic values because atoms such as theirs 

 can only be linked with a simple bond. 



Later researches have confirmed this. The univalent 

 halogens give, like hydrogen, constant atomic refractions, 

 both in the elementary state and in their compounds. The 

 multivalent elements, on the other hand, such as oxygen 

 and nitrogen, display different optical values, according to 

 the kind of bond. 



In the course of such researches the behaviour of oxygen 

 as a quadrivalent element, which had been previously con- 

 jectured, was established with certainty, and afterwards 

 confirmed synthetically by Collie, Tickle, and others. 



The theory which accounted for the optical abnormalities 

 of certain classes of bodies, making them, in fact, abnor- 

 malities no longer, has proved extraordinarily fruitful. It 

 formed the starting point of all subsequent discoveries in 

 the subject, and, indeed, we may describe the progress of 

 this branch of science during the last twenty-five years as 

 based essentially on this conception. 



For not until we had fathomed the mystery of the 

 benzene refractive increment 6 was it possible to know 

 for certain that the variable valency of the multivalent 

 elements is always of determining influence on the optical 

 behaviour of bodies. Thus for the first time a spectro- 

 chemical method was called into being for the study of 

 chemical struclure, and the foundations were laid of what 

 we now rail " sjjcrl I'ui hcmi^try. "' 



We niu'.t now niLirn unn- more to the formula for 

 refractivity. Newton's expression f(n- — i)/;/]? had proved 

 not constant for the temperature in the case of fluid bodies, 

 and was, therefore, replaced by Gladstone and Dale's 

 more satisfactory ratio [(n— i) 'i/]P. For twenty years and 

 more this did admirable service. .\s, however, the number 



of observations kept on increasing, even this formula be- 

 trayed imperfections which finally led to its abandonment. 

 It is 'impossible here to follow the argument in detail, and 

 we must be content with the remark that comparisons of 

 bodies in different states of aggregation failed to yield 

 satisfactory constants. The values of [(«— i)/(i]P for a 

 fluid or solid substance always came out considerably 

 greater than for the same substance in the state of gas or 

 vapour. 



Then by a happy chance two physicists, L. Lorenz, of 

 Copenhagen, and H. A. Lorentz, of Leyden, came forward 

 simultaneously in 1880 with a new expression for refrac- 

 tion. One of them started from the ordinary theory of 

 light, the other from Maxwell's electromagnetic theory of 

 light based on Faraday's views, and they both reached the 

 same result, viz. that the true measure of refractivity is 

 furnished by the expression 



\;!' + 2.'d 

 Experimental tests showed that this theoretical expression 

 was, in fact, for all bodies, practically unaffected not only 

 by temperature and pressure, but also by the state of 

 aggregation. 



Chemical tests confirmed the utility of the new optical 

 standard, since the operation of all the laws before men- 

 tioned was observed to be even more exact when the new 

 constant was applied. 



Moreover, the expression for refraction proved valuable 

 in another respect. It was found to be very suitable for 

 measuring the dispersh'e power of bodies. 



If ;;,. and n,. denote the refractive indices for the limits 

 of the visible spectrum, i.e. for violet and for red light, 

 the difference of the refractivities for these end-rays of the 

 spectrum, 



\ll^- + 2 «r'^ + 2/</ 



is the measure of the power of different bodies to disperse 

 light — to broaden out the spectrum. This ratio proved to 

 be constant as regards temperature, pressure, and state of 

 aggregation. 



Gladstone had already observed that dispersion, like re- 

 fraction, was connected with the chemical nature of bodies. 

 Quantitative relations were, however, only obtained when 

 a constant for refractivity had been found. And then from 

 the molecular dispersions of compounds the atomic dis- 

 persions of their elements were deduced. 



We cannot enter here into the relations which were thus 

 shown to exist between the chemical composition of sub- 

 stances and their power to disperse light. We need only 

 remark that the case as a whole is analogous to that of 

 refraction. Dispersion is, however, a still more sensitive 

 and more constitutional property, and therefore in many 

 cases it is specially adapted as an aid to research on 

 chemical structure. 



It only remains to add a few remarks on the applications 

 of spectrochemistry in science and in practical life. 



It has already been shown the principles on which 

 spectrochemical methods of examination in general can be 

 applied to the solution of scientific problems, to the dis- 

 covery of the chemical structure of single substances or 

 whole classes of bodies. 



Now there is a large number of substances, some of 

 them artificially built up by synthesis out of their elements, 

 some of them occurring in the vegetable and animal 

 kingdoms, or even in inorganic nature, the structure of 

 which is of remarkable delicacy and instability. Among 

 them are, for instance, the so-called " tautomeric " com- 

 pounds, hydrogen peroxide, and many other unstable com- 

 pounds. Substances of this kind are of a very special 

 interest, for in consequence of their tendency to change, 

 thev are the principal cause of metamorphoses, the un- 

 ceasing circulation of matter, the eternal birth and decay 

 that go on in nature. 



Research in the atomic structure of such bodies by 

 purely chemical methods is often very difficult, and not 

 seldom impossible, because, owing to their sensitive 

 organisation, chemical interference leads either to changes 

 in the grouping of the atoms, which cannot always be 

 controlled, or even to total decomposition. 



NO. 1859, VOL 72] 



